The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
One particular type of blind rivet is known as a multigrip rivet. A typical rivet of this type comprises an outer tubular shell having circumferential grooves spaced at intervals along its length, and a mandrel, having a radially enlarged head at one end, extending through the shell. The shell also usually has a radially extending head, which acts against one face of a workpiece. To set the rivet, the mandrel is pulled axially relative to the shell, while supporting the shell head, causing the enlarged head of the mandrel to urge against the opposed end face of the tubular shell. The circumferential grooves in the tubular shell are thereby caused to collapse in an axial direction, securing the workpiece components together, and finally the portions of the shell between the grooves are caused to expand radially to form a characteristic “cottage loaf” shape, or “cottage loaf set”. The rivet thereby holds the workpiece components together and fills the hole in the workpiece through which it passes.
In the prior art, one type of multigrip rivet has a tubular shell with a profiled exterior shape in which there are two axially spaced regions of reduced diameter. The reduced diameter regions are produced by deforming the shell inwardly, after the mandrel has been inserted into the shell, by means of a crimping operation (i.e. using radially inwardly moving dies) or a rolling operation.
U.S. Pat. No. 5,496,140 describes another known blind rivet. It comprises an outer tubular shell, and a mandrel extending therethrough. In this case the outer shell is provided with one or two sets of recesses, the recesses of each set being arranged in a circumferential direction, spaced apart from each other, and essentially in the same axial position. The bottoms of the recesses form a secant with respect to the cylindrical circumference of the rivet shell.
Blind rivets are often required to operate in oversized holes, and in a variety of grip thicknesses. Some of the known blind rivets can perform well in such conditions, the multigrip nature of the “cottage-loaf” shape deformation also being capable of adjusting to oversize holes, and the variety of grip thicknesses; this being achieved by appropriate positioning of the circumferential grooves or recesses in the shell. However, particularly when operating at the extremes of grip thickness or oversize holes, groove or recess depth is critical; if the grooves are too deep they will cause cracking or even fracture of the rivet body; but if the grooves are too shallow they will not provide sufficient resistance to prevent the mandrel head from pulling into or even through the mandrel body. These problems are exacerbated where the workpiece materials to be secured are soft (since the grooves need to be deeper in the rivet shell to prevent mandrel head pull-through), or friable (since the rivet is more likely to promote cracking or fracture when set, especially if the mandrel head pulls into the rivet body), and/or if the surface of the rivet is coated with a low friction material such as PTFE (polytetrafluoroethylene), this sometimes being done to provide enhanced abrasion or corrosion resistance.
Cracking or fracture of the rivet shell is obviously undesirable since it results in a failed or at least a significantly weakened connection. Passage of the mandrel head into, or through the rivet shell is also undesirable. If the mandrel head pulls into the rivet shell it causes radial expansion of the rivet shell between the grooves, resulting in splitting of the workpiece material. If the mandrel head can pull completely through the rivet shell the result is that the mandrel head shaft portion can protrude beyond the flange giving a potentially hazardous condition.
We have observed that while the prior art blind rivets function in oversize holes and in a variety of grip thicknesses, they do not perform reliably when the conditions are exacerbated by the use of soft or friable materials as the workpiece components, or when the rivet itself is provided with a low surface friction coating.
We have further discovered that performance of all rivets, and particularly the performance of rivets when set in soft or friable workpieces, and the performance of low friction coated rivets in these and other workpieces, can be significantly improved by providing a rivet shell having sets of radial indentations rather than the circumferential groove or secant shaped recesses of the prior art.